Field of the Invention
The present invention relates to a transmission electron microscope.
Description of Related Art
In a transmission electron microscope, an objective lens is used as a lens for focusing and imaging an electron beam. A magnetic objective lens assembly is disclosed as one type of such an objective lens, for example, JP-A-2005-32588.
However, where a magnetic sample that is susceptible to the effects of a magnetic field is observed with a transmission electron microscope, there is the problem that the magnetic properties of the sample are affected by the magnetic field of the objective lens. That is, when a sample susceptible to the effects of a magnetic field is placed under the influence of the magnetic field produced by an objective lens, the intrinsic state of the sample may not be observed.
Therefore, the objective lens assembly forth in JP-A-2005-32588 has a first magnetic lens and a second magnetic lens for producing magnetic fields near a sample placement region which extends along the optical axis and in which a sample is to be placed. The objective lens assembly is so configured that, in the sample placement region, the magnetic fields produced by the first and second magnetic lenses, respectively, cancel out to zero.
In an electron microscope, electron lenses made of a ferromagnetic substance such as iron are used. An optical system for the microscope is constituted by stacking such electron lenses on top of each other. Accordingly, a magnetic circuit is formed over the whole electron microscope. Therefore, it follows that the effects of magnetic fields arising from other than the objective lens reach the surroundings of the sample. For this reason, if the magnetic fields produced by the first and second magnetic lenses of the objective lens assembly set forth in JP-A-2005-32588 cancel out to zero, the effects of magnetic fields produced other than from the objective lens assembly reach the surroundings of the sample.
For example, during observation using an electron microscope, the excitation of the imaging lens system is varied such as a variation in the magnification. This in turn varies the magnetic field. The effects of this variation may reach the surroundings of the sample via the magnetic circuit.
It is estimated that the magnitude of the effects of magnetic fields produced from other than the objective lens assembly, i.e., the magnitude of the effects of stray magnetic fields at the position of the sample, is on the order of hundreds of μT (microteslas). If there is a weak magnetic field on the order of hundreds of μT in this way, and if a material with high magnetic permeability (χ is approximately 106) is used, the problem cannot be neglected. If the magnetic field is as weak as hundreds of μT, displacement of magnetic domains or variations in the morphology of magnetic domains may occur in a magnetic substance with high magnetic permeability. The principal purpose of analysis of a magnetic substance is morphological observation of a magnetic domain distribution inside the magnetic substance. To permit this observation, surrounding magnetic fields need to be made as low as possible. Also, movement of magnetic domains and morphological variations in the magnetic domains should be avoided.